1) Output static libraries from the C++ code to be consumed by SLINK/SLINK64.
2) Compile the Fortran code to DLLs and link the C++ code with MS tools.

Both will involve a fair bit of effort to rework. My questions are;
a) Am I missing something here?
b) Are there any updates to SCC / SLINK64 in the near future that mean I should wait instead of embarking upon a rewrite?

My preferred approach would be to create a Microsoft C++ DLL exporting extern "C" functions. Then link this to a Fortran project that uses C_EXTERNAL for the imported functions. SLINK64 will be able to load the 64 bit C++ DLL. It will also work for the 32 bit case using SLINK.